Current mirror circuits have recently been considered for use in reducing current imbalance in parallel LED strings. The lifetime of LED devices is sensitive to operating currents. If LED devices are arranged in parallel strings, for example, as a means to increase power and light output, the slight differences among LED devices would cause current imbalance among LED strings, therefore affecting the light uniformity and lifetime of the overall LED system. There are many current balancing techniques. However, the new concept of self-configurable and re-configurable current mirror circuits that do not require using a predetermined current reference and a separate power supply to provide a current reference was disclosed in PCT publication WO 2012/095680. A typical embodiment of such circuits is shown in FIG. 1.
For current mirror circuits, it is necessary to choose a current reference for other current sources to follow. Where current mirror circuits are used for parallel LED strings, the current source with the smallest current should be chosen as the current reference. In the circuit shown in FIG. 1, selection switches in the form of S-transistors are included in the current mirror circuit, which also includes Q-transistors in respective parallel current sources. FIG. 2 shows a version of the current mirror circuit in FIG. 1 in more detail. The circuit in FIG. 2, which has been practically confirmed, includes an auxiliary circuit to select the current source with the smallest current as the current reference, and thus select which S-transistor to close. An improved version of this self-reconfigurable current mirror circuit incorporates an opamp assisted circuit, as shown in FIG. 3. The power supply for the opamp circuit can be derived from a simple circuit, as shown in FIG. 4 for a 2-string system.
However, while the circuits in FIG. 2 and FIG. 3 work well under normal situations, the circuits will fail to operate when one of the strings suffers an open circuit fault. It should be noted that a short circuit fault of one device within a LED string only increases current imbalance, and will not cause the current mirror circuit to fail.
FIG. 5 highlights the open circuit problem when the last string of the circuit of FIG. 3 suffers an open circuit fault, which is marked as a cross “x” in FIG. 5. Under this open circuit fault, the electric potential Vin3 at point A of the circuit is not floating, since the transistor S3 is still conducting. The voltage at point A will fall to a very low value because the base-collector of the bipolar junction transistor (BJT) Q3 conducts through the diode action of the base-collector of S3. The low current though this base-collector of S3 is small and so is the voltage drop across the resistor RE of the faulty current string.
Consequently, the voltage at point A will be very low. It will be equal to the sum of the voltage of the collector-emitter voltage of transistor Q3 and the voltage across RE of the faulty string. Since RE is a resistor with a low resistance value (typically a few ohms) and the current coming from the base-collector diode of S3 is small, the voltage across RE of the faulty string is also very small. Such a low voltage at point A will mislead the current mirror detection circuit to wrongly select this faulty string as the current reference. FIG. 6 shows the practical measurements of the three currents of the circuit in FIG. 5, with each of the three LED current strings suddenly cut off to simulate an open circuit fault. It can be seen that the three currents drop to near zero.